Directional couplers are passive devices typically used in radio frequency applications to couple part of the transmission power or energy in a transmission line by a known amount out through another port. Often the coupling is achieved by using two transmission lines set close enough together such that energy passing through one line is coupled to the other line. Designers of directional couplers often need to determine a mechanical layout of these transmission lines to accomplish a preselected degree of coupling. Often this preselected degree of coupling is 3 dB or less and constrains the designer to position the lines very close together, which can create manufacturing and/or fabrication yield problems. More specifically, in some cases, the designer can be constrained by the rules associated with a design tool for laying out the transmission lines.
Conventional directional couplers can include interdigitated coupling segments positioned on a flat surface. U.S. Pat. No. 3,516,024 to Lange describes such an interdigitated strip line coupler. A variation of the Lange coupler is described by Waugh and LaCombe in an IEEE article. (Waugh, R., LaCombe, D.: “‘Unfolding’ the Lange Coupler”, IEEE Trans., 1972, MTT-20, pp. 777-779). These conventional couplers can however be difficult and expensive to manufacture in some circumstances. In addition, the performance of these conventional couplers can be limited.